The other day as I was doing some research on Srimati Radharani, for her appearance day post, I ran across the following;

…Oṁkāra is a combination of the letters a, u and m.A-kāreṇocyate kṛṣṇaḥ the letter a (a-kāra) refers to Kṛṣṇa, who is sarva-lokaika-nāyakaḥ, the master of all living entities and planets, material and spiritual. Nāyaka means “leader.” He is the supreme leader (nityo nityānāṁ cetanaś cetanānām (Kaṭha Upaniṣad 2.2.13)). The letter u (u-kāra) indicates Śrīmatī Rādhārāṇī, the pleasure potency of Kṛṣṇa, and m (ma-kāra) indicates the living entities (jīvas). Thus oṁ is the complete combination of Kṛṣṇa, His potency and His eternal servitors. In other words, oṁkāra represents Kṛṣṇa, His name, fame, pastimes, entourage, expansions, devotees, potencies and everything else pertaining to Him. As Caitanya Mahāprabhu states in the present verse of Śrī Caitanya-caritāmṛta, sarva-viśva-dhāma: oṁkāra is the resting place of everything, just as Kṛṣṇa is the resting place of everything (brahmaṇo hi pratiṣṭhāham) (CC Adi 7.128, Purport)

…The Lord has innumerable incarnations, and oṁkāra is one of them. As Kṛṣṇa states in Bhagavad-gītā: “Amongst vibrations, I am the syllable om.” (Bg. 9.17) This means that oṁkāra is nondifferent from Kṛṣṇa. Impersonalists, however, give more importance to oṁkāra than to the Personality of Godhead, Kṛṣṇa. The fact is, however, that any representational incarnation of the Supreme Lord is nondifferent from Him. Such an incarnation or representation is as good spiritually as the Supreme Lord. Oṁkāra is therefore the ultimate representation of all the Vedas. Indeed, the Vedic mantras or hymns have transcendental value because they are prefixed by the syllable om. The Vaiṣṇavas interpret oṁkāra as follows: by the letter O, Kṛṣṇa, the Supreme Personality of Godhead, is indicated; by the letter U, Kṛṣṇa’s eternal consort Śrīmatī Rādhārāṇī is indicated; and by the letter M, the eternal servitor of the Supreme Lord, the living entity, is indicated. Śaṅkara has not given such importance to the oṁkāra. However, importance is given in the Vedas, the Rāmāyaṇa, the Purāṇas and in the Mahābhārata from beginning to end. Thus the glories of the Supreme Lord, the Supreme Personality of Godhead, are declared. (Teachings of Lord Caitanya, Chapter 20)